An internal combustion engine of a vehicle may be stopped and started during a vehicle drive cycle to conserve fuel. The engine may be stopped while a requested driver demand torque is low so that the vehicle may coast or so that the driver demand torque may be met via a motor. The stopped engine may be restarted if driver demand torque increases or if a battery is to be charged. However, for vehicles that are not propelled via an electric motor and that include an automatic transmission, it may be difficult to stop the engine in response to low driver demand torques because stopping the engine stops operation of a mechanically driven pump in the transmission that supplies pressurized fluid to activate transmission gears. In some transmissions, an electrically driven pump may supply transmission fluid within the transmission if the mechanically driven pump is stopped, but electrically driven pumps are often flow limited based on transmission packaging. Consequently, electrically driven pumps lack capacity to shift through a plurality of transmission gears in a short time period. Consequently, an expected level of transmission performance may not be maintainable when the engine is stopped even if the transmission includes an electrically driven pump. Therefore, it may be desirable to provide a way to stop an engine and maintain transmission operation during low driver demand conditions so that engine fuel consumption may be reduced during low driver demand torque conditions and while the vehicle is stopped.
The inventors herein have recognized the above-mentioned issues and have developed a method for operating a vehicle driveline, comprising: activating an electrically driven transmission pump in response to a request to stop an engine; and adjusting a position of a hydraulically operated screw driven clutch gap filling device via fluid provided via the electrically driven transmission pump.
By adjusting a gap filling device of a transmission clutch, it may be possible to provide the technical result of reducing flow of transmission fluid to operate transmission clutches when an engine coupled to the transmission is stopped rotating. Specifically, a small amount of transmission fluid may be supplied to a device that alters a transmission clutch filling volume so that the transmission clutch closes to engage a transmission gear using a smaller amount of transmission fluid. Consequently, the transmission clutch may be operated with transmission fluid flow from an electrically driven pump having less transmission fluid flow capacity than an engine driven transmission fluid pump. Further, in some examples, a plurality of gap filling devices may be adjusted so that a transmission gear may be applied or engaged via a plurality of clutches with reduced gap volumes. In this way, it may be possible to operate the transmission with an electrically driven pump that has a lower fluid flow capacity.
The present description may provide several advantages. In particular, the approach may reduce vehicle fuel consumption and improve response to a driver requesting driveline torque when an engine in the driveline is stopped rotating. Further, the approach may provide for partially engaging a plurality of gear clutches when an engine coupled to a transmission is stopped so that the transmission may be engaged in an appropriate gear for applying engine torque to vehicle wheels even as vehicle speed changes while the engine is stopped. Additionally, the approach may provide desired shifting even when the transmission electric pump lacks flow capacity to close a transmission clutch in a desired amount of time.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.